Ca facilitates secondary metabolism
Apart from the aforementioned primary metabolism process, three
secondary metabolism related proteins were activated by exogenous Ca
(Table 1), whose abundance increased significantly at high Ca level
compared with low Ca or medium Ca treatment. The involved metabolites
included methylglyoxal, vitamin, isoprenoid and turpentine.
Lactoylglutathione lyase (spot 6) participates in the detoxification of
methylglyoxal, and a higher transcription of lactoylglutathione lyase
has been reported in aluminum treated tomato
root.49 Methylglyoxal
is a by-product of a number of metabolic pathways, especially
glycolysis.50 Plant
usually maintains low level methylglyoxal under normal growth conditions
while abiotic stresses induce dramatically increase of methylglyoxal,
which functions as a toxic molecule and inhibits a variety of biological
processes in plant, such as seed germination, photosynthesis and root
growth.51 High Ca
treatment induced higher expression of lactoylglutathione lyase possibly
accelerated the detoxification and remove of excessive methylglyoxal,
which is an unavoidable by-product of Ca improved various metabolism
processes, such as glycolysis, in P. massoniana seedlings.
Both sopentenyl diphosphate isomerase (IDI)-I and IDI-II participate in
the rate-limiting step for the biosynthesis of terpenoid compounds,
which are important components of turpentine in pinaceae
plant.52 In the present
study, Ca deficiency down-regulated the expression of IDI- II like (spot
12) while high Ca supply recovered its expression to normal level (Table
1). Adequate exogenous Ca supply possibly promoted the biosynthesis and
accumulation of turpentine in P. massoniana through restoring IDI
mediated terpenoid biosynthesis, which is of production guidance
significance in P. massoniana forest industry for turpentine
yield.
Thiamine thiazole synthase involves in the biosynthesis of thiazole,
which is the precursor of thiamine (vitamin
B1).53 The significant
up-regulated thiamine thiazole synthase 2 (spot 63) implied high
concentration of exogenous Ca benefited various glycometabolism and
energy pathways may be related to the enhanced biosynthesis of thiamine,
as thiamine is an essential cofactor for the enzymes activating a plenty
of carbohydrate metabolism
pathways.53
The mechanisms underlying high Ca treatment resulted strengthened
secondary metabolism could be attributed to the following potential
factors. Primarily, abundant exogenous Ca promoted the processes of
photosynthesis and diverse primary metabolism pathways, which laid a
sound material and energy foundation for the anabolism of secondary
metabolites. Besides, the expression of some secondary metabolism
related proteins is a response to high Ca motivated primary metabolism,
with the purpose of removing excessive by-products of primary metabolism
or synthesizing cofactors for primary metabolism.
In addition, based on the difference in Ca requirement for plants and Ca
concentrations in natural habitats, plants can be classified into
calcifuge plant and calcicole
plant.16 It can be
deduced that P. massoniana is a calcicole plant to some extent
considering the improved various material metabolism pathways,
facilitated photosynthesis, increased biomass and better growth at high
Ca level.